Slice onions. Dice celery. Drain oysters and save liquid. Oysters may be chopped or left whole as desired. Add water to oyster liquid to make 1 litre In stock pot, combine bay leaf, one onion, and bay leaf and simmer for about 1 hour uncovered. Remove from heat; allow to cool for about 1 hour, strain, and set aside. In butter, saute remaining onion and celery; cook for about 5 minutes or till pale in color. Add flour and stir well, but DO NOT BROWN. Add some strained oyster stock: stir well tp prevent lumps from forming. Add remaining stock and heat till hot and thickened (about 10-15 minutes). Add oysters and cream. Heat about 5 minutes more. Add sherry and ladle into serving bowls and garnish with chopped parsley.